Circadian rhythms and environmental lighting regulate a number of endocrine and behavioral functions. Dispersed chick pineal cells remain rhythmic and responsive to light in culture. Light has two effects on the melatonin rhythm: acute suppression and entrainment (phase shifts) of the underlying clock. The key enzyme in melatonin synthesis, serotonin N-acetyltransferase was partially purified, separated and distinguished from a related, drug-metabolizing, enzyme. Psychoactive tricyclic drugs (e.g., clomipramine) inhibited melatonin synthesis, possible by blocking calcium influx. Caffeine, at concentrations that release calcium from intracellular stores, induced light-like phase shifts in the melantonin rhythm while increasing melatonin synthesis, and suggested a role for intracellular calcium mobilization in photoentrainment.